Method of applying a firmly adherent coating of non-metallic substances to an electrically conductive substrate

ABSTRACT

Process for coating an electrically conductive substrate with a coating, a portion of which comprises a powdered non-metallic substance which comprises: 1. Applying to said electrically conductive surface powder of a non-metallic substance by electrophoresis whereby a porous coating is obtained; 2. Thereafter compressing the so coated electrically conductive surface at a pressure of at least 1kg p.sq.cm; and 3. Subjecting the so compressed coated electrically conductive surface to electroplating in an electrolytic bath containing a metal ion or a plurality of metal ions whereby the coating obtained in step 1 by electrophoresis is solidified galvanically by the deposition of said metal or said metals on said substrate and within the pores of said porous coating.

United States Patent 11 1 Heyes 1 51 Dec. 30, 1975 [76] Inventor: Josef Heyes, Niederkasseler Strasse 1 l4, 4 Dusseldorf, Germany 221' Filed: Mar. 18, 1974 211 Appl. 190.: 452,160

[30] Foreign Application Priority Data Mar. 16, 1973 Germany 2313104 [52] US. Cl 204/16; 204/181 [51] Int. Cl. C25D 5/02 [58] Field of Search 204/16, 181

[56] I References Cited UNITED STATES PATENTS 2,424,140 7/1947 Beecher 204/16 2,858,256 10/1958 Fahnoe et al. 3,537,960 11/1970 Esola et a]. 204/16 FOREIGN PATENTS OR APPLICATIONS 2,000,407 7/1971 Germany 204/16 Primary ExaminerT. M. Tufariello Attorney, Agent, or FirmBurgess, Dinklage & Sprung [57] ABSTRACT Process for coating an electrically conductive substrate with a coating, a portion of which comprises a powdered non-metallic substance which comprises:

1. Applying to said electrically conductive surface powder of a non-metallic substance by electrophoresis whereby a porous coating is obtained;

2. Thereafter compressing the so coated electrically conductive surface at a pressure of at least 1kg p.sq.cm; and

3. Subjecting the so compressed coated electrically conductive surface to electroplating in an electrolytic bath containing a metal ion or a plurality of metal ions whereby the coating obtained in step 1 by electrophoresis is solidified galvanically by the deposition of said metal or said metals on said substrate and within the pores of said porous coating.

13 Claims, No Drawings METHOD OF APPLYING A FIRMLY ADHERENT COATING OF NON-METALLIC SUBSTANCES TO AN ELECTRICALLY CONDUCTIVE SUBSTRATE BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of applying a firmly adherent coating of a non-metallic substance to an electrically conductive substrate, in which a coating of a powdered, non-metallic substance is applied by the use of electrophoresis to the electrically conductive layer and the so coated electrically conductive layer is thereafter subjected to an electrolytic galvanic coating process in which a metal or an alloy is deposited within the pores of the powder coating and onto the electrically conductive substrate.

DISCUSSION OF THE PRIOR ART Processes for the deposition of non-metallic substances onto an electrically conductive substrate have been known. One such process is described in German Offenlegungsschrift Pat. No. 2,011,966, the disclosure of which is hereby incorporated herein by reference. In these processes, a powder layer of a non-metallic substance is deposited upon an electrically conductive substrate by electrophoresis. Subsequently thereto, the so coated material undergoes an electroplating process. It has been known to maintain the pores disposed be tween the powder particles deposited on the electrically conductive substrate free of air or gas by means of the suspending agent of the electrophoresis. This suspending agent is left in the pores. Another method of maintaining the pores of the powder layer free of air or gas is to occupy such porous area by the electrolytes which are to be put into the pores by means of a wetting agent. It is also known to use vacuum and, in some cases, pressure to accomplish the desired result.

In the same process, layers having a predetermined porosity have been created by providing the powdered substances, prior to the electrophoretic deposition, with a cover layer, preferably an organic cover layer. This layer must consist of a substance which can be removed after the electroplating, preferably by means of a solvent.

The non-metallic substances which can be applied electrophoretically are substantially carbides, nitrides, borides, oxides and silicides of metals, particularly tungsten, titanium, tantalum, molybdenum, niobium, zirconium, silicon and boron. It is also known to electrophoretically deposit diamond powder and other substances, such as aluminum trioxide, onto electrically conductive substrates.

In all cases, great hardness and resistance to wear are principal properties of these non-metallic substances. Unfortunately, when the metallic binding agents which are applied by the electroplating process are present, as is necessarily the case, the desired hardness and resistance to wear is somewhat impaired. Stated differently, while the non-metallic substances possess great hardness and resistance to wear, these properties are not possessed to the same extent by the metallic binding agents. Thus, it has become desirable to provide a coated electrically conductive substrate where the coating consists of both non-metallic and metallic particles which are improved with respect to hardness and wear resistance. It has become particularly desirable to improve the overall qualities of the coating on the electrically conductive substrate.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to increase the hardness and wear resistant qualities of firmly adhered coatings applied by means of non-metallic powders and metallic binding agents to electrically conductive substrates. It is another object of the present invention to provide a highly wear resistant and hard firmly adherent coating on an electrically conductive substrate provided by use of an electrophoretic process followed by an electroplating process.

In accordance with the present invention, it has been found that improved hardness and wear resistant properties can be provided by a process for coating an electrically conductive substrate with a coating, a portion of which comprises a powdered non-metallic substance, which process comprises:

l. Applying to said electrically conductive surface powder of a non-metallic substrate by electrophoresis whereby a porous-coating is obtained;

2. Thereafter compressing the so coated electrically conductive surface at a pressure of at least lkg p.sq.cm; and

3. Subjecting the so compressed coated electrically conductive surface to electroplating in an electrolytic bath containing a metal ion or a plurality of ions, whereby the coating obtained in step 1 by electrophoresis is solidified galvanically by the deposition of said metal or alloy on said substrate and within the pores of said porous coating.

The fundamental solution to the long-standing problems in the prior art resides in increasing the ratio of powdered non-metallic substance to the metallic binding agent applied during the electroplating process.

' This can be understood considering as an example of a non-metallic powder tungsten carbide, and as an example of metallic binding agent applied by electroplating, cobalt.

The pores of the tungsten carbide powder having an average grain size of about 1 micron, which is to be deposited by electrophoresis, are relatively large. After determining the bulk weight of the powder to be used in the depositing process to be approximately 4.4 in the pycnometer, it has been possible to estimate its pore capacity. Since the density of the tungsten carbide in the massive state is 15.7, only 28 percent of the pycnometer capacity was filled with it. This high percentage of pore space that is not filled with the metallic binder permits only a limited amount of tungsten carbide in the tungsten carbidecobalt layer, say 30 percent by volume.

The amount of tungsten carbide in the tungsten carbide-cobalt layer can be markedly increased if the process is conducted by diminishing the thickness of the electrophoretically applied and dried powder layer under elevated pressure. Thus, at the heart of the present invention there is a provision whereby the percentage of the non-metallic powder in the layer bonded by electroplating is increased by diminishing the thickness of the electrophoretically applied and dried powder layer under elevated pressure. This reduces the pore space before the electroplating takes place and provides a layer, following the electroplating, which has a higher non-metallic concentration. Generally speaking, the concentration of the non-metallic material will be between 30 and percent by volume based upon the combined amounts of nonmetallic substance and metal binding agent.

Theprocess of the present invention is conducted by applying pressure to the electrophoretically applied and dried powder layer. Preferably, a pressure of at least 1kg p.sq.cm is applied. Mostsuitably, this pressure should be between and 4000 kilogrammes.

To exert this pressure, presses are mainly used. It is preferred that an isostatic press be employed which acts on all sides of the layer, although the pressure may be exerted in some other manner, for example, by rolling. The pressure is to amount in some cases to as much as several tons per cm.

DESCRIPTION OF PREFERRED EMBODIMENTS The successful electroplating of a greatly compressed non-metal powder layer applied electrophoroetically to an electrically conductive substrate, resulting in great strength of adhesion, is surprising. It is decidedly different from the hard metal technique in which a mixture of non-metal powders and metallic powders are compressed and subjected to sintering. This process is furthermore limited to the preparation of self-supporting bodies, while the new process of the invention permits the coating of bodies of any kind with hard metals or the like.

It is often of great importance that the hard, wearresistant coatings prepared in accordance with the invention, which preferably contain more than 60 percent of the hard powder, also have a sufficient thickness after the compression and electroplating. It is, therefore, an important additional feature of the invention to apply electrophoretically a plurality of coatings successively, and to compress them only after the total thickness of the applied layers is sufficient for the thickness desired after compression, say between 0.01 and 0.2 mm.

In this case, it'is desirable each time to turn on the voltage in the organic liquid and deposit a comparatively thin layer of powder on the object to be coated, then to turn off the voltage, remove the object from the bath, and dry the coating, and then to re-immerse the object, repeating this procedure as often as desired.

In the case of a plurality of layers the procedure may be to apply a number of layers, compress them, and then apply additional layers and again compress them.

It has also been found that, with this application of a plurality of layers, the tendency for cracks and holes to form is substantially less than in the case of a thick coating'applied in one step. While not wishing to be bound by any theories, such cracks and holes are probably the consequence of surface tensions and/or of the stronger evolution of hydrogen during the more protracted single electrophoretic application of a thicker powder layer on the conductive substrate. This hydrogen cannot escape from the already deposited powder layers and will not result in cratering of the layer upon a further increase in the'hydrogen pressure. When the powder layer or layers are compressed, they can be protected on the substrate by a covering, preferably by a metal foil.

In carrying out the process of the invention utilizing a plurality of separate electrophoresis coating processes, it is preferred that each electrophoresis coating be conducted such that a powder layer of no more than 0.1 mm is provided prior to such time as the layer is dried. Stated differently, if a layer of substantial thickness is desired, it is preferred that this layer be provided percentage of the non-metallic coating material to be contained in the final adherent coating.

In the electroplating process, it is otherwise generally customary to rinse the objects immediately after the degreasing and pickling operations and then place them without delay into the electroplating bath. But in the process of the invention, a period of time elapses between the electrophoretic layer-wise application of the non-metallic powder and the electroplating operation, owing to the steps which must be performed between them, and this makes it necessary to pay atten' tion to certain details.

In particular, the passivation of the metallic substrate should be prevented, because the firm adhesion of the metallic binding agent which is to be applied by electroplating cannot be achieved on a passivated substrate. Therefore, the substrate should normally consist of a metallic material which will not oxidize with the formation of a passive coating on its surface when the electrophoretically applied powder is dried and air gains access to the pores in the powder layer. If, however, the substrate should be made of a metallic material which in the course of the process would form a passive coating steel, for example it will be necessary to apply to its surface a thin coating of a metallic material which does not undergo passivation. These metallic materials, which under the given circumstances have the character of noble metals, include specifically copper, silver, gold, and platinum.

In the process of the invention, it may also be advantageous before or during the electroplating procedure to free the compressed powder layer of any air or gas remaining in the fine pores. Practical methods of accomplishing this are:

To reduce the surface tension of the electrolyte and wetting agent.

To electroplate at reduced pressure over the electro- Iyte.

To place the workpieces in an evacuable chamber which is coated with an electrolyte after evacuation in order to perform the galvanizing procedure.

The electroplating itself is to be performed in an electrolyte having a pH value of preferably 4.5 to 6.0 so as to be still sufficiently high for the depositing of the metal. The current density is to be 0.2 to l A/dm.

. In the process of the invention it is furthermore possible advantageously to bring about a temporary consolidation of the powder particles applied in layers.

That is to say, the electrophoretically applied layers are soft and, for example, can easily be wiped off. Also, in the time that elapses between the electrophoretic application and the compression, the electrically conductive substrate may become passive so that the adhesion of the galvanically deposited metal becomes inadequate.

The temporary consolidation of the powder applied in layers, which shall be called cementing hereinafter, can be accomplished in various ways and with different classes of substances.

In general, care must'be taken to see that thecementing of the powder does not result in contamination of the electrically conductive substrate-One of=the methods of achieving such consolidation consists in the application of a suitable varnish to the, powder coating, for example by immersing the coating in the varnish or by spraying the varnish onto the.-coating.,Examples of varnishes for this purpose include:

After the compression of the powder coating cemented with varnish, the varnish must be eliminated prior to the electroplating. If the substrate is sufficiently thermostable, as is the case with many metals and alloys, for example, the varnish may advantageously be burned off. This does not interfere with the electroplating that follows. There is also theadded advantage that, as a result of the temperature rise in the combustion of the varnish, a drying takes place and a transformation of metal hydroxide formed during the electrophoretic coating to the corresponding metal oxide, thereby accomplishing a consolidation of the powder coating.

The removal of varnish can also be performed by means of a solvent suitable for the varnishj this should preferably be done shortly prior to the electroplating operation.

Another excellent method of cementing the powder coating prior to compression consists in the use of solutions or emulsions of organic adhesives, such as especially collodion, starch, gelatine and substances similar thereto.

With solutions of this kind, which need to contain only a small concentration of adhesive less than percent for example and usually 1 to 9 percent by weight it is not only possible to achieve an effective cementing, but also an advantage that may be useful in many cases lies in the fact that such adhesives do not interfere with the galvanic deposition on the electrically conductive substrate.

It has been found, for example, that a collodion coating on a brass sheet, produced from an 0.5 percent collodion solution, or a starch or gelatine coating formed from a solution of similar concentration, did not interfere with the deposition of cobalt from a cobalt electrolyte, for example.

The object to be electroplated can thus be electroplated without removing such cements. However, in the case of such cements one can remove them prior to electroplating in order, for example, to prevent them from getting into the electrolyte. The usual solvents can be used for this purpose, e.g., hot water in the case of starch. Here again it is advantageous to remove the cement shortly before electroplating.

It is also possible to coat the nonmetallic particles with an extremely thin metallic coating by electroless metallizing the particles by the solutions well known for this purpose.

Thereby, the quantity of particles deposited by a following electrophoresis is increased. In this manner, a layer of 0.1 may be deposited within two minutes.

In order to more fully illustrate the invention and manner of practicing the same, the following example is presented:

EXAMPLE I. A steel plate was provided with an adherent, thin coating of a noble metal, namely, copper.

2. An electrophoretic coating was performed in 1 liter of alcohol plus I 1000 mol of cobalt chloride plus 250 g, of tungsten carbide HC 100 with a grain size of approximately ,1 millimicron, at a field strength of 14 volts per .centimeter. The specimen was coated for one minute each time, "then removed from the alcoholic solution and dried, and then immersed 'for another minute in the solution to give the alcohol timeto penetrate into the pores; then the current was turned on again and off again after a minute and the whole procedure was repeated.

- After ten repetitions, 18.9 milligrams of tungsten carbide had been deposited on each square centimeter of the surface of the-specimen.

3. The specimen was then dried at about 300C. The coated plate was then dippedinto a 5 percent starch solution containing a wetting agent (TEXAPON, registered trademark of Deutsche Hydrierwerke AG, Dusseldorf)l Immediately following this treatment the specimen was dried and placed between two thin sheets of rubber and the latter were cemented. The sandwich was compressed in an isostatic press at a pressure of 3 tons per square centimeter.

- 4. After the compression the specimens were placed for 180 minutes in a cobalt sulfate bath. The bath contained:

504 g/l cobalt sulfate 17 g/l sodium chloride 45 g/l boric acid and 2 ml of a wetting agent.

The temperature of the bath during the electroplating was about 20C, the pH value about 5.0, the current density about 0.5A/dm The strength of adhesion of the coating obtained was excellent. After a light lapping of the coating the hard metal coating proved to be uniform and dense.

The invention thus solved the problem of producing hardmetal-like sandwiches with a correspondingly high content of non-metallic substances such as carbides, without the use of sintering or the like.

The substrates to be coated may have any desired, complex configuration. The coating is firmly adherent, and its strength is achieved byelectroplating, that is, without the use of sintering or other such heat-expending processes.

Generally speaking, the process parameters for each electrophoretic coating step are as follows:

Distance between cathode and anode 2 to 20 centimeters, field strength between 5 and volts per centimetre.

Generally speaking, the following metals can be electroplated so as to form firm adherent, hard, wear resistant coatings in combination with non-metallic coatings deposited electrophoretically:

copper, silver, nickel, cobalt, bronce.

What is claimed is:

1. A process for coating an electrically conductive substrate with a coating comprising a powdered nonmetallic hard substance bonded by a metal, the thickness of the coating being a multiple of the thickness of a particle of the powdered substance which comprises:

l. applying to said electrically conductive surface powder of a nonmetallic substance by electrophoresis whereby a porous coating is obtained;

2. thereafter compressing the so coated electrically conductive surface at a pressure of at least 1 kg per sq. cm.; and i 3. subjecting the so compressionably coated electrically conductive surface to electroplating in an iron, zinc, brass,

electrolytic bath containing a metal or metal alloy whereby the coating obtained in step 1 by electrophoresis is solidified galvanically by the deposition of said metal or alloy on said substrate and within the pores of said porous coating.

2. A process according to claim 1 wherein said compression is effected by applying pressure to the porous coating isostatically and the pressure at each point is between and 4000 kg per square centimetre.

3. A process according to claim 2 wherein step 1 is repeated a multiplying of times before step 2 is performed whereby a plurality of powder layers are ap plied electrophoretically one over the other.

4. A process according to claim 3 wherein following an electrophoresis coating of said substrate with said porous coating, said coating is heated at a temperature of between 100 and 500C. to effect transformation of metal hydroxide formed during the electrophoretic coating to the corresponding metal oxides.

5. A process according to claim 4 wherein said heating to effect transformation follows the last electrophoretic coating.

6. A process according to claim 4 wherein after the final electrophoretic coating and prior to said compression to the electrophoretic coating there is applied an adhesion or varnishing.

7. A process according to claim 6 wherein an adhesive is utilized which adhesive is selected from the group consisting of collodion starch or gelatin and no adhesive removal step prior to electroplating is utilized.

8. A process according to claim 6 wherein a varnish is employed and follows said compression but prior to said electroplating said varnish is removed.

9. A process according to claim 8 wherein said varnish is removed by heating at temperature sufficient to vaporize said varnish.

10. A process according to claim 1 wherein an electrolyte is used for the electroplating which has a pH value of 4, 5 to 6, at which pH hydroxide of the metal to be deposited is not dissolved in the electrolyte.

11. A process according to claim 10 wherein said electrically conductive substrates surface contains a non-passivatable metallic substance.

12. A process according to claim 11 wherein said non-passivatable metallic substance is selected from the group consisting of copper, silver, gold, platinum.

13. A process according to claim 1 wherein the particles of a nonmetallic substance is coated by electroless metallizing by means of solutions well known, for this purpose previous to the electrophoresis. 

1. A PROCESS FOR COATING AN ELECTRICALLY CONDUCTIVE SUBSTRATE WITH A COATING COMPRISING A POWDERED NONMETALLIC HARD SUBSTANCE BONDED BY A METAL, THE THICKNESS OF THE COATING BEING A MULTIPLE OF THE THICKNESS OF A PARTICLE OF THE POWDERED SUBSTANCE WHICH COMPRISES:
 1. APPLYING TO SAID ELECTRICALLY CONDUCTIVE SURFACE POWDER OF A NONMETALLIC SUBSTANCE BY ELECTROPHORESIS WHEREBY A POROUS COATING IS OBTAINED;
 2. THEREAFTER COMPRESSING THE SO COATED ELECTRICALLY CONDUCTIVE SURFACE AT A PRESSURE OF AT LEAST 1 KG PER SQ, CM.; AND
 2. thereafter compressing the so coated electrically conductive surface at a pressure of at least 1 kg per sq. cm.; and
 2. A process according to claim 1 wherein said compression is effected by applying pressure to the porous coating isostatically and the pressure at each point is between 10 and 4000 kg per square centimetre.
 3. A process according to claim 2 wherein step 1 is repeated a multiplying of times before step 2 is performed whereby a plurality of powder layers are applied electrophoretically one over the other.
 3. subjecting the so compressionably coated electrically conductive surface to electroplating in an electrolytic bath containing a metal or metal alloy whereby the coating obtained in step 1 by electrophoresis is solidified galvanically by the deposition of said metal or alloy on said substrate and within the pores of said porous coating.
 3. SUBJECTING THE SO COMPRESSIONABLY COATED ELECTRICALLY CONDUCTIVE SURFACE TO ELECTROPLATING IN AN ELECTROLYTIC HATH CONTAINING A METAL OR METAL ALLOY WHEREBY THE COATING OBTAINED IN STEP 1 BY ELECTROPHORESIS IN SOLIDIFIED GALVANICALLY BY THE DEPOSITION OF SAID METAL OR ALLOY ON SAID SUBSTRATE WITHIN THE PORES OF SAID POROUS COATING.
 4. A process according to claim 3 wherein following an electrophoresis coating of said substrate with said porous coating, said coating is heated at a temperature of between 100* and 500*C. to effect transformation of metal hydroxide formed during the electrophoretic coating to the corresponding metal oxides.
 5. A process according to claim 4 wherein said heating to effect transformation follows the last electrophoretic coating.
 6. A process according to claim 4 wherein after the final electrophoretic coating and prior to said compression to the electrophoretic coating there is applied an adhesion or varnishing.
 7. A process according to claim 6 wherein an adhesive is utilized which adhesive is selected from the group consisting of collodion starch or gelatin and no adhesive removal step prior to electroplating is utilized.
 8. A process according to claim 6 wherein a varnish is employed and follows said compression but prior to said electroplating said varnish is removed.
 9. A process according to claim 8 wherein said varnish is removed by heating at temperature sufficient to vaporize said varnish.
 10. A process according to claim 1 wherein an electrolyte is used for the electroplating which has a pH value of 4, 5 to 6, at which pH hydroxide of the metal to be deposited is not dissolved in the electrolyte.
 11. A process according to claim 10 wherein said electrically conductive substrate''s surface contains a non-passivatable metallic substance.
 12. A process according to claim 11 wherein said non-passivatable metallic substance is selected from the group consisting of copper, silver, gold, Platinum.
 13. A process according to claim 1 wherein the particles of a nonmetallic substance is coated by electroless metallizing by means of solutions well known, for this purpose previous to the electrophoresis. 